1. Field of the Invention
This invention relates to a process for the improvement of the flow characteristics of slurries of solid fuels and mineral matter in water. More particularly it is concerned with a process for the production of coal-water slurries which are made better Bingham plastics by the incorporation therein of a prescribed group of additives as hereinafter more fully described. Thereby reduced viscosities are obtained at high rates of applied shear. This fact in turn is advantageous both by virtue of the savings in pumping energy which are effected as well as by virtue of the longevity which is imparted to the pumping equipment involved.
Most solid carbonaceous fuels, as mined, generally contain varying amounts of water, which in some instances may be as high as 40 wt. % or even higher in the case of low grade solid fuels. This water is an undesirable constituent of the fuel, particularly in the case of fuels of high water content. Thus a slurry containing 50 wt. % water and 50 wt. % solid fuel would contain considerably less than that amount of fuel when the fuel is measured on a dry basis.
Furthermore, when coal is transported in slurried form any excess amount of water merely reduces the transportation efficiency.
The amount of water necessary to form a pumpable slurry depends on the surface characteristics of the solid fuel. For example, soot formed during the partial oxidation of a carbonaceous material has such a high surface area that a concentration of such soot in water in excess of a few wt. % renders the resulting slurry unpumpable. In the case of a slurry which is to be fed to a gas generator, it is necessary that the solid fuel be ground to such an extent that a major portion thereof will pass through a 200 mesh sieve so that the particles are substantially completely converted to oxides of carbon during their short residence time within the gasification zone. However, ordinarily before reaching the gasification zone the slurry must pass through various pieces of equipment such as heat exchangers and compressors on its way from the slurry zone to the gas generation zone. Accordingly the slurry must be pumpable but in the case of a slurry made up of solid fuel particles most of which will pass through a 60 mesh sieve it has been found that ordinarily a pumpable slurry must contain from about 55 to 60 wt. % water. Unfortunately a slurry containing this amount of water renders the operation of the gasifier unsatisfactory as this excessive amount of water moderates the temperature of the reaction zone to such an extent that it seriously affects its thermal efficiency. It has been found that the optimum amount of water in a solid fuel-water slurry which may be used as feed to a gas generation zone will lie from between 40 and 50 wt. %. A water content of 30 wt. % would be even more preferable, if it could be achieved. However, in the case of such aqueous slurries, the viscosity is remarkably increased with a loss in flowability if the coal concentration is increased beyond a certain point. On the other hand, if the concentration of the coal therein is reduced, both the transportation and the gasification efficiencies are lowered as already noted and an expensive dehydration step is required to recover dry coal.
Increase of coal content and reduction of the viscosity of a coal-water slurry are therefore mutually exclusive processes in ordinary circumstances although it would be desirable to achieve both.
Increase of the viscosity and reduction of the flowability in an aqueous slurry of coal are due to agglomeration of the coal particles in an aqueous slurry. Agglomeration is further aided by increased coal concentrations. The finer the particle size of dispersed coal powder, the better is the dispersion stability thereof. However, the cost of pulverization is increased if the degree of pulverization is enhanced. Finely divided coal now used in thermoelectric power plants has such a size that 80% of the particles will pass through a 200-mesh sieve.
It may be considered that if a surface active agent acting as a dispersant is added to an aqueous slurry of coal powder, the surface active agent will adsorb onto the coal particles and will exert the desirable functions of crumbling agglomerated particles and also preventing agglomeration of coal particles with the result that a good dispersion state will be attained.
It is therefore an object of this invention to produce solid fuel- water slurries having a relatively high solids content. Still another object of the invention is to produce pumpable slurries of solid fuel in water wherein the bulk of the solid fuel will pass through a sieve as fine as 60 mesh and in which the water content of the slurry will range between about 40 and 50 wt. %. Yet another object of the invention is to produce an improved Bingham plastic slurry, which by definition is handled with lower expenditure of energy as a result of lowered viscosity at high shear rates which may be expected at high rates of flow. Yet another object of the invention is to extend the useful life of pumping equipment by lowering the viscosity of the slurries pumped when operating at customary shear rates. Yet another object of the invention is to provide a cheap and uncomplicated method of coal-water slurry rheology control from readily available materials, which would be especially beneficial in areas where prior art viscosity controlling agents such as wood derived lignosulfonates are in short supply or not available. These and other objects will become apparent on further reading of this specification.